But the SDC said the government had reported a 6.3% decrease in carbon emissions from its offices since the year 1999-2000, an insufficient reduction for it to hit a target of 12.5% reductions by 2011-12.

And it was not nearly enough to contribute to the legally binding national goal to cut emissions by 80% by 2050, the commission said.

The annual Sustainable Development in Government report also said future targets for environmental performance by departments, to replace those that will shortly expire, must be "set high" to reflect the scale of the challenge ahead.

A failure to cut carbon will force departments to buy carbon credits to cover their emissions, under new rules coming in next year to promote energy and carbon savings, the SDC said.

While the government remains on track to meet targets for levels of recycling and electricity from renewables, both fell last year, the report showed. And the amount of electricity used was on the rise, largely due to increased use of IT and the need for more air conditioning of server rooms.

The SDC welcomed commitments in the budget to support the shift to low-carbon vehicles, but said the government's current travel arrangements allowed staff to claim mileage at rates above fuel costs, providing "perverse" incentives for car use. Instead they should be given incentives to use more sustainable forms of travel or travel less, it urged.

The commission also said controls should be put on officials flying domestically and to European destinations where rail travel was an option. And there needed to be greater understanding of the full environmental impacts of government operations, for example developing "water footprints" and investigating the carbon footprint of supply chains of goods bought by departments.

Rebecca Willis, the vice-chairwoman of the SDC, said: "The government has taken some really significant steps making its own operations more sustainable. But it is still not on track to meet crucial targets, including reducing carbon emissions from offices."

William Jordan, central government's chief sustainability officer, said: "Significant progress has been made by the government in delivering on its commitment to deliver sustainable operations on the government's own estate."

He said he looked forward to working with the SDC to address the issues raised in the report and said the government would be revising targets and commitments on sustainability this year to ensure they reflected "leading practice".